1. Field of the Invention:
This invention relates to the detection of gases produced in subterranean oil and gas wells during drilling operations and the monitoring of gas levels to detect incipient gas kicks.
2. Description of the Prior Art:
In conventional drilling operations, a drilling fluid or drilling mud is continuously circulated between the drill bit and the surface of the well. Minute quantities of gas may be entrained in this circulating drilling fluid and delivered to the surface where they can be identified by a gas detector. In conventional gas detection systems, a motor driven vaccum pump draws air through a gas trap mounted in the drilling fluid flow line and the trapped gas is transported to an on-site laboratory. The gas is then passed through a filter, a flow meter and a hot wire bridge type gas detector having two filaments maintained at different temperatures to make it possible to distinguish between wet and dry gas. Conventional instruments which are manually operated, can detect as little as 0.01 cubic feet of gas per hour mixed with the air drawn from the trap. The gas detector can be connected to a recording apparatus so that the signal appears on the recorder chart. Absolute values of gas concentrations cannot be obtained in this manner since the amount of air mixed with the fluctuating levels of gas in the circulating fluid does not remain constant. Relative changes in the level in the drilling fluids can, however, be detected by an operator. With these conventional systems, the operator must also continuously compensate for changes in sensitivities of the sensors and must adjust his gas detector system based upon the relative levels of gas detected. For example, some conventional sensor units have an upper limit on the gas concentration at which they can be employed while other sensor elements can only detect relatively high gas concentrations. It is therefore conventional practice for gas detection systems to employ multiple sensors for use in different flow ranges. The operator must therefore determine which sensor is to be employed to give an accurate detection of gas levels. Since absolute gas concentrations cannot be obtained with conventional systems, the operator's choice of sensor levels is subject to interpretation and to error.
Another prior art method of employing conventional manual sensor systems is to use a single sensor capable of detecting relatively low gas concentrations and to dilute the gas sample with ambient air to reduce the absolute level of the sensor signal. Corresponding range changes to compensate for variable manual dilution must also be made in order to monitor the gas levels present. Such changes make it difficult to determine the exact levels of hydrocarbon gases present in the circulating drilling fluid, and the driller will therefore be unsure if he has encountered a gas pocket and if a gas kick is imminent. These random changes in signal level can also make a permanent record of gas level difficult to interpret.